Peel the marrow and cut in half lengthways.
Remove the seeds and sprinkle with salt and pepper.
Melt the butter in a pan, add the onions and mushrooms and saute until the onion is transparent.
Add the remaining ingredients.
Spoon onto the marrow halves.
Wrap each in foil with an opening at the top.
Put into the Slo cooker side by side and pour 150ml (1/4 pt) boiling water around.
Cook for the recommended time.
Lift out carefully and remove the foil.
Drain the remaining liquid from the Slo cooker.
Return the two halves of marrow to the Slo cooker.
Arrange slices of tomato down the centre of each half and sprinkle with grated cheese.
Remove the cooking pot from the Slo cooker and brown under a hot grill.